The present invention provides a method for reducing noise in a non-contact gauge measurement system utilizing structured light. In particular, the present invention provides a method to facilitate inspection of prismatic objects having uncoated surfaces, such as turbine or compressor blades, using a combination of object orientation and polarized light in an optical three-dimensional structured light gauge measurement system.
Traditionally, gauge measurement of a manufactured object having a complex three dimensional surface such as an airfoil (e.g. compressor blade) is a tedious and time consuming process. Airfoils, including forged blades such as those used on aircraft engines, electrical power generators, and the like, are inspected for deformations which may include, but are not limited to, skew, twist, scaling, and translation. More specifically, airfoils are inspected for deformation parameters such as platform orientation, contour cross-section, bow and twist along a stacking axis, thickness, and chord length at given cross-sections.
One method of obtaining dense and accurate digital data representing these parameters for an individual airfoil is through use of a coordinate measuring machine (commonly known as a xe2x80x9cCMMxe2x80x9d). CMM""s translate and rotate a sensor probe into contact with the surface of an object undergoing testing to sample the position of various points on the object""s surface. Before a sensor probe may be brought into contact with an object, the object must be secured in a known physical position and orientation, such that a set of known reference points may be established. For airfoil measurement, six physical contact points are utilized, defining a xe2x80x9csix point nestxe2x80x9d. This set of six data points establish the position and orientation of the airfoil in its physical holder and enable the contact points to be translated to any other coordinate system. CMM""s provide high quality (i.e. highly accurate) measurements of the sample points. However, the time to scan an airfoil is relatively slow as the process of positioning the airfoil in the six point nest is time-consuming, and the sensor probe must be continually repositioned to obtain data. Once the high quality surface points are collected, software processes these points into deviations from values generated in a computer assisted drawing (CAD) model of the object and analyzes the deviations in terms of process-based shape deformations. Current CMM processing software, however, is also relatively slow.
An alternate method of obtaining measurements representing deformation parameters of an object employs full-field non-contact range sensors. Non-contact full-field sensors can quickly scan the external surfaces of opaque objects, using laser or white light, significantly faster than CMMs. Examples of non-contact sensors include sensors that project laser line gratings onto the surface of an object and process detected images thereof using stereo triangulation; and those based on single laser line scan plus rotation of the object. Additional non-contact sensors are based on phase-shifted moir xc3xa9 patterns and white light. While these sensors are capable of scanning the part quickly and obtaining large quantities of data, the level of accuracy is affected by undesirable reflections of the scan light from shiny or prismatic surfaces on the object.
To compensate for these undesired reflections, shiny or prismatic surfaces on the object are traditionally coated with a diffusing material such as a paint or powder to non-contact gauge measurement. This additional step adds uncertainty to the measurement, and increases measurement time which is highly undesirable. Accordingly, there is a need for eliminating or reducing the effect of undesired reflections from shiny or prismatic object surfaces in a non-contact measurement system in a manner which does not require application of diffuse coatings to surfaces of the object being tested.
Briefly stated, the present invention provides a method for the inspection of shiny metal prismatic objects having uncoated prismatic surfaces, such as compressor blades, using polarized light in an optical three-dimensional structured light measurement system.
In one embodiment of the present invention, a structured light measurement system projects a structured light pattern onto the surface of an object, parallel to a plane bisecting one or more prismatic features of the object. An imaging system receives the structured light pattern reflected from the surface of the object and analyzes the deformation of the reflected light pattern to calculate the surface features of the object. The projected light is polarized at a known polarization angle, and the reflected light is polarized to a related angle, such that light reflected directly and indirectly from the planar surfaces of the object is separated by the imaging system. Using multiple images of the object obtained with polarized light, difference images and region masks can be generated to reduce or eliminate undesired reflections and noise from the resulting images of the object.
The foregoing and other objects, features, and advantages of the invention as well as presently preferred embodiments thereof will become more apparent from the reading of the following description in connection with the accompanying drawings.